The point is that historical anecdotes about the past climate, such as the claim that Greenland used to be green, or that Newfoundland (Vinland) was full of grapes, have to be treated with caution.

For starters, the accuracy of some historical claims is questionable&colon; it is not clear that Vinland of Viking sagas refers to modern-day Newfoundland, or even that there really were grapes, for instance.

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Even when historical records are accurate, their interpretation is not as straightforward as many assume. Take the frost fairs held in London when the River Thames in England froze over, which are sometimes hailed as proof of how cold it was during the so-called Little Ice Age (see We are just recovering from the Little Ice Age). The slowing of water flow by the old London Bridge is now seen as a crucial factor in the freezing of the river, which explains why the lower reaches of the river did not freeze in 1963, even though it was the third-coldest winter in England since 1659 and parts of the river upstream of London did freeze.

Growth bands and coral

To work out how the average global temperature has changed over the centuries, climate scientists need long-term records from as many different parts of the world as possible. Historical records do not provide this, which is why they have turned to other indicators such as growth bands in trees and corals.

These proxy records have their problems too&colon; tree rings can reflect the effects of rainfall as well as temperature, for instance. The uncertainties also become greater the further back you look, as the evidence becomes sparser. And there are also very few proxies from the southern hemisphere, so most reconstructions are of northern hemisphere temperature only.

There are a dozen or so temperature reconstructions for the northern hemisphere that go back beyond 1600, including the so-called “hockey stick” (see Climate myths&colon; The ‘hockey stick’ graph has been proven wrong). These studies suggest there were periods of unusual warmth from around AD 900 to AD 1300, but details vary widely in each reconstruction.

What matters most

In the southern hemisphere, the picture is even more mixed, with evidence of both warm and cool periods around this time. The Medieval Warm Period may have been mostly a regional phenomenon, with the extremes reflecting a redistribution of heat around the planet rather than a big overall rise in the average global temperature.

What is clear, both from the temperature reconstructions and from independent evidence – such as the extent of the recent melting of mountain glaciers – is that the planet has been warmer in the past few decades than at any time during the medieval period. In fact, the world may not have been so warm for 6000 or even 125,000 years (see Climate myths&colon; It has been warmer in the past, what’s the big deal?).

What really matters, though, is not how warm it is now, but how warm it is going to get in the future. Even the temperature reconstructions that show the greatest variations in the past 1000 years suggest up until the 1980s, average temperature changes remained within a narrow band spanning 1ºC at most. Now we are climbing out of that band, and the latest IPCC report (pdf format) predicts a further rise of 0.5ºC by 2030 and a whopping 6.4ºC by 2100 in the worst case scenario.